(a) Field of the Invention
The present invention relates to a so-called projector-type head lamp, and more particularly to a projector-type head lamp for vehicles having improved light distribution characteristics.
(b) Description of the Prior Art
The projector-type head lamp for vehicles is required for a light distribution pattern which brightly illuminates the road surface in front of the car without dazzling the driver of a car running on the opposite lane when passing each other. As a head lamp having a light distribution pattern meeting such requirements and of which the lens configuration is simple and the entire shape can be made small, so-called projector-type head lamps have been proposed. A typical one of such projector-type head lamps comprises a lamp bulb, operable as a light source, having a filament, a reflector partially having an elliptic reflecting surface which has a first focus near the light source and a second focus in front of the light source, a shade located near the second focus of the reflector, and a convex lens so formed as to have its focus near the second focus of the reflector and to transmit in the direction of radial optical axis the rays of light emitted from the light source, reflected by the reflector and shaped by the shade. In the projector-type head lamp having an elliptic reflecting surface in a part of the inner reflecting surface as described above, the filament, as the light source, takes the form of an, elongated cylinder in practice, and can be disposed either parallel to the direction of the optical axis of the reflector or perpendicularly to the optical axis. Since the light distribution pattern should preferably be wider horizontally than vertically, the filament is disposed horizontally in a direction perpendicular to the optical axis of the reflector. The illuminated areas defined when the road surface is illuminated by such a projector-type head lamp is schematically shown as areas, each of which is enclosed with a closed curved line in FIG. 1 (FIG. 1 shows the keep-to-the-left traffic system). In FIG. 1, the reference numeral 11 indicates the shoulder of the subject car's lane, 12 the shoulder of the opposite lane, 13 a center line, and 14 the course of the subject car. The optical axis of the reflector of the head lamp is generally directed to the subject car's course. The three closed curved lines 15a, 15b and 15c each form each an isolux line; the area enclosed by the curved line 15a is a central area in which the illuminance is very high (hot zone); and the curved line 15c diagramatically shows a profile of the illuminated area. In a light distribution pattern formed by a conventional reflector partially having an elliptic reflecting surface, namely, an illuminated area, the lower center of the profile line 15c is indented in the direction of the driving course of the car as indicated by the reference numeral 16, so that the illumination thus obtained is not satisfactory.
The reason why such an indentation or dark area develops will be explained below with reference to FIGS. 2 to 4. Each point of the reflector can be approximated by a flat small mirror, but since each point of the reflector is contributed to the production of the filament image on the screen, the total illuminance obtained with the head lamp is considered to be due to the total superposition of individual filament images from all the points of the reflector. For example, FIGS. 2 (A1), (B1) and (C1) show the positions of the typical points l, m and n, respectively, on the reflector having a spheroidal reflecting surface, FIGS. 2 (A2), (B2) and (C2) show the filament images l', m' and n', respectively, reflected by the typical points l, m and n onto the screen, and FIGS. (A3), (B3) and (C3) show the positions of the typical points l, m and n as well as the shapes l", m" and n" of the filament images l', m' and n' reflected by the typical points l, m and n onto the screen. The filament images at the points on the reflector (except for the area near the apex in which the opening for fixation of the lamp bulb is to be installed because this area does work a reflector) vary in orientation and shape from one point to another as shown in FIG. 3. As seen from FIG. 3, the filament images are generally elongated and horizontal in the longitudinal reflecting area of the reflector crossing the vertical plane in which the optical axis lies, and, as they move apart from the optical axis, become smaller horizontal images. In the lateral reflecting area of the reflector crossing the horizontal plane in which the optical axis lies, the filament images are small horizontal images which become increasingly more contracted horizontally as their distance from the optical axis increases. Also it will be seen from FIG. 3 that in the reflecting area defined by the line of intersection between the reflecting surface and the horizontal plane in which the optical axis lies and the line of intersection between the reflecting surface and the vertical plane in which the optical axis lies, namely, in the upper and lower right and left areas, the filament images are oblique As the filament images reflected at the points on the reflector shown in FIG. 3 are superposed on each other, the area enclosed with a dash line in FIG. 4 generally defines the profile of the superposed images. The cause of the aforementioned indentation is that the filament images in the left and right areas 20A and 20B below the optical axis of the reflector as viewed from the light source as shown in FIG. 3 and in the left and right areas 21A and 21B above the optical axis of the reflector are greatly slanted. These areas are shown as generally square ones defined as enclosed with a dot-dash line for the simplicity of explanation. FIG. 4 schematically show an enlarged scale of the filament images reflected at eight typical points in the left and right areas 20A and 20B below the optical axis of the reflector as viewed from the light source. As seen, indentatations develop at two places indicated with the reference numerals 22A and 22B. Actually, the indentation or dark area 22B has no problem since it can be cut off by the shade disposed between the reflector and convex lens and also it is located beyond the illuminated area. However, the indentation 22A is problematic because it takes place at the side of the illuminated area nearest the vehicle and it does not contribute to the effective illumination. The illuminated area or the profile line thereof should preferably have the shape of the area enclosed with the dot-dash line as shown in FIG. 1. The illuminated area should desirably have a pattern with the side nearest the vehicle being nearly horizontal and not indented, as shown with the reference numeral 15d.